[PATCH v2 2/5] mm/swapfile: use percpu_ref to serialize against concurrent swapoff

From: Miaohe Lin <linmiaohe@huawei.com>
To: <akpm@linux-foundation.org>
Cc: <ying.huang@intel.com>, <dennis@kernel.org>,
	<tim.c.chen@linux.intel.com>, <hughd@google.com>,
	<hannes@cmpxchg.org>, <mhocko@suse.com>, <iamjoonsoo.kim@lge.com>,
	<alexs@kernel.org>, <david@redhat.com>, <minchan@kernel.org>,
	<richard.weiyang@gmail.com>, <linux-kernel@vger.kernel.org>,
	<linux-mm@kvack.org>, <linmiaohe@huawei.com>
Subject: [PATCH v2 2/5] mm/swapfile: use percpu_ref to serialize against concurrent swapoff
Date: Sat, 17 Apr 2021 05:40:36 -0400	[thread overview]
Message-ID: <20210417094039.51711-3-linmiaohe@huawei.com> (raw)
In-Reply-To: <20210417094039.51711-1-linmiaohe@huawei.com>

Use percpu_ref to serialize against concurrent swapoff. Also remove the
SWP_VALID flag because it's used together with RCU solution.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 include/linux/swap.h |  3 +--
 mm/swapfile.c        | 43 +++++++++++++++++--------------------------
 2 files changed, 18 insertions(+), 28 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 8be36eb58b7a..993693b38109 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -177,7 +177,6 @@ enum {
 	SWP_PAGE_DISCARD = (1 << 10),	/* freed swap page-cluster discards */
 	SWP_STABLE_WRITES = (1 << 11),	/* no overwrite PG_writeback pages */
 	SWP_SYNCHRONOUS_IO = (1 << 12),	/* synchronous IO is efficient */
-	SWP_VALID	= (1 << 13),	/* swap is valid to be operated on? */
 					/* add others here before... */
 	SWP_SCANNING	= (1 << 14),	/* refcount in scan_swap_map */
 };
@@ -514,7 +513,7 @@ sector_t swap_page_sector(struct page *page);
 
 static inline void put_swap_device(struct swap_info_struct *si)
 {
-	rcu_read_unlock();
+	percpu_ref_put(&si->users);
 }
 
 #else /* CONFIG_SWAP */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 66515a3a2824..90e197bc2eeb 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1279,18 +1279,12 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
  * via preventing the swap device from being swapoff, until
  * put_swap_device() is called.  Otherwise return NULL.
  *
- * The entirety of the RCU read critical section must come before the
- * return from or after the call to synchronize_rcu() in
- * enable_swap_info() or swapoff().  So if "si->flags & SWP_VALID" is
- * true, the si->map, si->cluster_info, etc. must be valid in the
- * critical section.
- *
  * Notice that swapoff or swapoff+swapon can still happen before the
- * rcu_read_lock() in get_swap_device() or after the rcu_read_unlock()
- * in put_swap_device() if there isn't any other way to prevent
- * swapoff, such as page lock, page table lock, etc.  The caller must
- * be prepared for that.  For example, the following situation is
- * possible.
+ * percpu_ref_tryget_live() in get_swap_device() or after the
+ * percpu_ref_put() in put_swap_device() if there isn't any other way
+ * to prevent swapoff, such as page lock, page table lock, etc.  The
+ * caller must be prepared for that.  For example, the following
+ * situation is possible.
  *
  *   CPU1				CPU2
  *   do_swap_page()
@@ -1318,21 +1312,24 @@ struct swap_info_struct *get_swap_device(swp_entry_t entry)
 	si = swp_swap_info(entry);
 	if (!si)
 		goto bad_nofile;
-
-	rcu_read_lock();
-	if (data_race(!(si->flags & SWP_VALID)))
-		goto unlock_out;
+	if (!percpu_ref_tryget_live(&si->users))
+		goto out;
+	/*
+	 * Guarantee we will not reference uninitialized fields
+	 * of swap_info_struct.
+	 */
+	smp_rmb();
 	offset = swp_offset(entry);
 	if (offset >= si->max)
-		goto unlock_out;
+		goto put_out;
 
 	return si;
 bad_nofile:
 	pr_err("%s: %s%08lx\n", __func__, Bad_file, entry.val);
 out:
 	return NULL;
-unlock_out:
-	rcu_read_unlock();
+put_out:
+	percpu_ref_put(&si->users);
 	return NULL;
 }
 
@@ -2475,7 +2472,7 @@ static void setup_swap_info(struct swap_info_struct *p, int prio,
 
 static void _enable_swap_info(struct swap_info_struct *p)
 {
-	p->flags |= SWP_WRITEOK | SWP_VALID;
+	p->flags |= SWP_WRITEOK;
 	atomic_long_add(p->pages, &nr_swap_pages);
 	total_swap_pages += p->pages;
 
@@ -2507,7 +2504,7 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
 	spin_unlock(&swap_lock);
 	/*
 	 * Guarantee swap_map, cluster_info, etc. fields are valid
-	 * between get/put_swap_device() if SWP_VALID bit is set
+	 * between get/put_swap_device().
 	 */
 	percpu_ref_resurrect(&p->users);
 	spin_lock(&swap_lock);
@@ -2625,12 +2622,6 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
 
 	reenable_swap_slots_cache_unlock();
 
-	spin_lock(&swap_lock);
-	spin_lock(&p->lock);
-	p->flags &= ~SWP_VALID;		/* mark swap device as invalid */
-	spin_unlock(&p->lock);
-	spin_unlock(&swap_lock);
-
 	percpu_ref_kill(&p->users);
 	/*
 	 * We need synchronize_rcu() here to protect the accessing
-- 
2.19.1

